Dementia and mild cognitive impairment are common among U.S elderly, yet despite their immense and growing burden relatively little is known about characteristics which lead to cognitive decline. Recent evidence, both epidemiological and pathophysiological, has suggested a possible relation between abnormal sleep characteristics and cognitive impairment due to both vascular etiologies and Alzheimer's disease. However, understanding of this relation is in its infancy. We propose to explore the relation of abnormal sleep characteristics to risk of cognitive impairment over 15 years of follow-up utilizing data fro the Atherosclerosis Risk in Communities (ARIC) cohort and the Sleep Heart Health Study. ARIC is a population-based prospective epidemiologic cohort, which has had several follow-up visits, including a visit in 1996-1998 (Visit 4, ages 54-73) and an ongoing visit (ARIC Visit 5/Neurocognitive Study (ARIC NCS, 2011-2013, ages 69-88)). The Sleep Heart Health Study, which included a subset of ARIC participants, took place in (1995-1998), at approximately the same time as ARIC Visit 4. A total of 1,892 individuals participated in both ARIC Visit 4 and had in-home overnight polysomnography as part of the Sleep Heart Health Study. We hypothesize that that abnormal sleep characteristics (inclusive of measures of hypoxia and disordered breathing, sleep fragmentation, and sleep duration) in middle age are associated with risk of cognitive impairment later in life. Several cognitive phenotypes will be explored, utilizing comprehensive neurocognitive data presently being collected through ARIC-NCS. These include 1) clinical dementia and mild cognitive impairment, 2) cognitive decline as assessed by change in 3 cognitive tests administered at ARIC Visit 4 (1996-1998) and repeated in ARIC NCS (2011-2013), and 3) brain MRI markers of cerebral vascular disease (i.e. white matter hyper-intensity volume, number of lacunar infarcts) and Alzheimer's disease (i.e. hippocampal and gray matter volume). Results from the proposed study will have a profound impact on the understanding of relations between sleep in middle-age and cognition in later life. Abnormal sleep characteristics and cognitive impairment are both highly prevalent among older adults, and abnormal sleep characteristics are amenable to modification through existing interventions. Therefore, elucidating whether abnormal sleep characteristics play a role in the etiology of cognitive impairment is of vital public health importance.